Grinnell Fire Protection was purchased by Tyco in 1976. Tyco bought Simplex Time Recorder Company on January 5, 2001 for US$1.15 billion and merged it with Grinnell Fire Protection, forming SimplexGrinnell. It is currently the largest fire protection company in the world.

Grinnell already knew Henry S. Parmalee, who patented the first automatic fire sprinkler head in 1874. Providence Steam & Gas partnered with Parmalee and manufactured the Parmalee sprinkler; Grinnell also designed and erected the piping installations into which the Parmalee sprinkler heads were fitted. Grinnell improved Parmalee's first practical automatic sprinkler and patented his own Grinnell sprinkler in 1882.[3] Continual improvements resulted in the glass disc sprinkler in 1890. With slight modifications, this sprinkler head is still used in modern fire sprinkler systems; sprinklers are even called le Grinnells in France.[4] In 1892, Grinnell organized the General Fire Extinguisher Company, which was renamed the Grinnell Fire Protection Company after his death in 1919.

Meanwhile, Edward G. Watkins founded the Simplex Time Recorder Company in 1894. Watkins invented and patented one of the first practical time clocks, along with the synchronized clock systems seen in schools up until recent times. The company, located in Gardner, Massachusetts, purchased the IBM Time Equipment Division in 1958.[5] This purchase included IBM's fire protection division. Originally, Simplex's fire alarm line simply consisted of relabeled IBM devices and control panels (including the 4217 and possibly the 4201 series), but they began to introduce their own products starting in the early 1960s, including the 4245, 4246, and 4247 series control panels. Most of Simplex's audible notification appliances prior to the 1990s were relabeled Benjamin Electric, Federal Signal, Faraday, and Autocall devices, such as the 2901-9806 (Federal Signal 450) 2901-9833 (Faraday 6020), and the 4050 (Federal Signal). Simplex's time division began to lose market share, but their newly founded fire alarm division significantly prospered. In 1970, Simplex introduced the 4208-series conventional control panels. The 4050-80 series was introduced as the signals for the 4208. Around this time, Simplex also introduced the iconic "T-bar" style pull station, which is still manufactured today. In 1975, Simplex introduced the 4207, a system similar in design to the 4208, but with updated features such as card-based modularity, DC battery trickle charging, compatibility with custom COP and read-back systems for pre-multiplex building control, and voice evacuation for large buildings. In 1977, Simplex introduced the Simplex 2903 series. This includes the 2903 light plates and the 2904 series remote lights. They also introduced their 2901-9833 and 2901-9806 horns that year. Two years later, in 1979, the Simplex 2903 strobe plate versions were introduced.

In 1979, Simplex introduced the LifeAlarm 2100, the industry's first networked multiplex building control system that was capable of handling fire alarm, security, HVAC, synchronized time, and watchman's tour. It centered around a large CPU console with built-in CRT display and keyboard that controlled the system's remote transponders (smaller self-sufficient panels connected to the MAPNET loop that distributed the system's I/O). In 1981, the 2100 was replaced with the popular and less-expensive 2120-series.[6] Unlike the 2100, the 2120's main CPU came in the form of a traditional wall-mount panel that was field-programmable, but several more options for remotely controlling the system through external CRT consoles and status command centers were now made available. The main panel featured a vacuum fluorescent display screen, DOS-based EEPROM programming, built-in printer, and a voice evacuation option. The 2120 was finally discontinued in 1992. These systems were possibly the first fire alarm systems with a screen display. Though flexible and robust, these systems were often criticized for being difficult to operate and troubleshoot. These two computer-based systems were accompanied by the conventional 2001-series. First released in 1978, the 2001 was a highly customizable system that used plug-in modules. Some were common to every system (basic controls), while many other options were available, including signal coding cards, waterflow monitor cards, and many others. These cards were also compatible with Simplex's multiplex systems. In 1988, the 2001 was discontinued in favor of the 4001 and 4002 series (which came out in 1985 and 1986, respectively), and the 1000-point 4100 addressable system which was introduced that year, along with the 4903 series light/strobe plates, which replaced the 2903/2904 series.[7] Around 1992, the 2120 was replaced with the 4120-series networked panel, and the 508-point 4020 addressable system was introduced at the same time, with the new 2099/4099 series T-Bar pull stations. In 1993, Simplex began manufacturing their 4903 mechanical horn strobe series, along with the 4904 and 4901 horns. In 1994, Simplex introduced the 4004, 4005, and 4009 NAC extenders. In 1998, the 4903 electronic horn strobe was released, and in 2001, Simplex introduced the TrueAlert-series, cited as being the industry's first addressable notification appliances which can be tested individually with a magnet without activating the entire system. The 4100U, a consolidated upgrade of Simplex's three largest addressable systems, followed in 2002, and was again updated in 2010 with the current 4100ES. [8] From time to time, SimplexGrinnell has used Cooper Wheelock, Gentex and Federal Signal notification appliances, mostly for outdoor/weatherproof applications. However, this is becoming less frequent with the introduction of weatherproof TrueAlert signals.[9][10]

SimplexGrinnell also markets burglar alarms, PA systems, and nurse call systems. They also provide testing, inspection, and maintenance services for fire alarm, sprinkler, suppression, security, and communication equipment. To a much lesser degree, the company also sells and services time clocks and master time systems.